A ratiometric output sensor is used to measure various physical parameters, such as pressure, strain or position. It is usually powered by a regulated DC supply; and its output is typically a voltage. The output voltage is not only a function of the environmental parameter that is measured; it is also a function of the DC supply (input) voltage in a predefined range. For this kind of sensor, it is the ratio of the sensor output voltage to the supply voltage, rather than the sensor output voltage itself, that indicates the value of the parameter measured by the sensor. The sensor is commonly located remotely from the control module and is powered thereby.
Ratiometric output sensors are typically powered by low voltage (e.g. 5 volts) regulated DC power supplies; and reverse voltage protection and over-voltage protection are typically provided by the DC power supply and/or the controller module in which the ratiometric sensor is used. But there is also a need for a self-protected ratiometric output sensor, particularly for mobile and/or battery operated applications. Reverse voltage protection is generally provided in electronic circuits by a series blocking diode, which permits current flow when the battery or other DC voltage supply is connected with a predetermined polarity but blocks current flow in the opposite direction when the polarity of the power supply connection is reversed from the predetermined polarity. But the typical blocking diode introduces a significant forward voltage drop in series with the sensor during sensor operation that is inconsistent with ratiometric output. Thus, there is a need for a power protected ratiometric output sensor circuit that provides reverse supply voltage protection without using a series diode in sensor operation and optionally provides over-voltage protection with minimal added part count and cost.